The present invention relates to a new and improved method of, and installation for, purifying liquids like wastewater by means of a filter bed containing aquatic plants planted in such filter bed.
In its more particular aspects, the present invention specifically relates to a new and improved method of, and installation for, purifying liquids like wastewater by means of a filter bed containing aquatic plants planted in such filter bed and in which method and installation the liquid to be purified is infed into the aquatic plant-containing filter bed through appropriately constructed inlet or infeed means. The purified liquid which outflows from the aquatic plant-containing filter bed, is received by appropriately constructed outlet or outfeed means.
Aquatic plant-containing filter beds of the aforementioned type have become known in the art during the past 10 to 20 years under various designations such as "Root Space Beds", "Aquatic Bed Purification Stages", "Reed-type Purification Installations" and so forth. In the following, such known aquatic plant-containing filter beds will be described briefly with respect to their construction and mode of operation.
In installations of the aforementioned type the liquid to be treated like, for example, wastewater substantially horizontally percolates or seeps through a soil matrix which is formed by the aquatic plant-containing filter bed. This soil matrix is provided with a secondary structure and thereby with high hydraulic conductivity due to the physical, chemical and biological activity of the roots and rhizoms of preselected aquatic plants. After a number of years, the hydraulic conductivity can assume values in the range of about K.sub.f =10.sup.-3 m/sec. Due to the continual mechanical and chemical activity and particularly due to continual mass exchange of the subterraneous organs or elements, the high permeability coefficients k.sub.f remain preserved even during continuous liquid infiltration or infeed. The aforementioned effects can connteract even an obstruction of the soil pores by means of solid materials entrained by the liquid to be purified.
The physical, chemical and biological processes which are determinant for the intended changes in the infiltrated liquid to be purified are irrelevant to the inventive control operations and, therefore, are here not discussed. These processes are described in detail in other publications, see, for example, applicant's German Patent Publications No. 2,944,421, published Nov. 3, 1979, and European Pat. No. 0,028,360, published Apr. 20, 1983; publication by R. Kickuth, H. J. Grommelt, entitled "Wurzelnahe Reaktionszonen in hydromorphen Boden", in Int. Sympos. Gumpenstein 1982, pages 681 to 688; German Patent Publication No. 3,406,004, published Feb. 20, 1984; German Patent Publication No. 2,418,979, published Apr. 19, 1974; publication by A. G. Boon, entitled "Report of a Visit by Members and Staff of WRC to Germany to Investigate the Root Zone Method for Treatment of Waste Waters", Water Research Processes, Aug. 1985; publication by L. Rodewald-Rudescu, entitled "Das Schilfrohr", in Die Binnengewasser, Vol. XXVII, Schweizerbartsche Verlagsbuchhandlung Stuttgart, 1974.
Generally, the body of soil which therefore is effective and forms the aquatic plant-containing filter bed, is sealed from the subsoil in any conventional suitable manner in order to block leakage of the liquid to be purified in a direction towards ground water and to ensure the substantially horizontal percolation or seepage which is typical for such purifying system, through the aquatic plant-containing filter bed.
The transport of the liquid to be treated in this manner is described by the transport equation ##EQU1## Therein v=k.sub.f (m/sec).times.dh/ds according to DARCY is the flow rate of the liquid along a hydraulic gradient dh/ds in a substrate having the permeability coefficient k.sub.f (m/sec) and the infiltration or infeed cross-sectional area .phi. (m.sup.2) for transporting or passing-through the liquid to be treated in this manner at a throughput Q (m.sup.3 /sec).
The coarse pore structure in the root area of the planted aquatic plants and which coarse pore structure is typical for the purification process and determinant for the transport of liquid, is formed from the given starting material due to restructuring and aggregating processes. In most cases the starting material constitutes a comparatively heavy soil containing significant proportions of fine and coarse clay.
The purification process employing aquatic plant-containing filter beds thus is distinctly different from all classic infiltration methods based on the transport capacity of a predetermined coarse-grained structure such as formed by coarse sand, gravel and so forth.
The initially mentioned methods and installations utilizing comparatively heavy soil which is aggregated and restructured for use as the aquatic plant-containing filter bed, achieve discrete activity increases, however, still possess significant operational disadvantages and still posses significant operational disadvantages and problems which will be explained in more detail hereinafter.
Generally, the biogenous restructurization and aggregation of the aquatic plant-containing filter bed by means of the aforementioned aquatic plants requires relatively long periods of time so that there may elapse four and, in some cases, even more years from the construction until the full activity of the installation.
Only after the aquatic plant-containing filter bed has been fully developed to its climax state due to the activity of the subterraneous organs, there can be expected permeability coefficients in the range of k.sub.f =10.sup.-3 m/sec whereupon the dimensions of such installations have been based with respect to the flow cross-sectional area .phi..
The permeability coefficient k.sub.f thus develops in the manner as illustrated in FIG. 5 and therefore does not permit, during the pre-phase of the operation, percolation or throughflow of the full liquid throughput Q for which the installation has been dimensioned.
If, however, the installation must accept the full liquid throughput Q from the start, which generally is the case in wastewater treatment, then, more or less considerable partial flows must be conducted away via the planum, i.e. the installation surface. Although such partial flow is subject to even favorable chemical and biological changes during contact with the surface of the aquatic plant-containing filter bed, such operational states or conditions are problematic for the following reasons:
(i) wastewater running off along the surface may cause malodorous burdens and aesthetically objectionable situations;
(ii) erosion grooves or flutes may be formed in the surface of freshly planted or still biogenously unstabilized filter beds and the medium to be treated or liquid to be purified flows nearly unchanged therethrough towards the outlet or outfeed;
(iii) the freshly planted or biogenously still unstabilized filter beds do not yet bring the full throughput capacity;
(iv) also the liquid to be purified and which liquid substantially vertically enters the aquatic plant-containing filter bed, may produce erosion phenomena, for example, dislocations of fine particles within the soil matrix of the aquatic plant-containing filter bed.
Furthermore and with fully developed installations of this type, there exists the problem that variable infeed rates of the liquid to be purified, for example, during dry phases and wet phases due to different rain water arrivals, cause variations in the hydromorphous condition and thus in the purification efficiency of the aquatic plant-containing filter bed.